1. Field of the Invention
The present invention relates to a head actuator for a magnetic disk drive unit.
2. Description of the Related Art
In a magnetic disk drive unit to be used as an external storage for a computer, a magnetic head is driven by a head actuator to perform tracking of the magnetic head on a magnetic disk rotatably driven in a steady state, thereby performing position control of the magnetic head in the radial direction of the magnetic disk. As a typical structure of the head actuator, there is known a rotary structure such that a carriage, on which the magnetic head is mounted, is rotated about a shaft. The optimization of such a structure has been searched for.
A conventional rotary head actuator comprises, for example, a carriage adapted to rotate to rotate about a shaft fixed to a housing and having an end on which a magnetic head is mounted, and a coil fixed to the carriage and receiving a driving force from a static magnetic field. The coil is usually located on the side of the actuator opposite to the magnetic head with respect to the shaft, and a wound portion of the coil parallel to the shaft crosses a radial magnetic flux produced by a permanent magnet to thereby generate a driving force of the head actuator. The permanent magnet is provided, for example, on the housing to which the shaft is fixed, and a driving direction of the head actuator depends on the direction of the magnetic flux generated by the permanent magnet and the direction of a current flowing in the coil.
Generally in such a magnetic disk drive unit, it is known that the larger the distance between the shaft of the head actuator and the magnetic head, the smaller the fluctuation in yaw angle (angle of the magnetic head to tracks on the magnetic disk) and the more advantageous in follow-up performance of the tracks in the case where the magnetic head is separated into an inductive head for writing and a magnetoresistive head (MR head) for reading (Japanese Patent Application filed by the present applicant on Dec. 7, 1993 and entitled "Magnetic Disk Drive Unit".
If a portion of the carriage on the side where the magnetic head is mounted is long, the weight distribution of the head actuator in its axial direction is unbalanced and causes a problem in that track slip, due to a mechanical external force, tends to occur. That is, if the center of gravity of the head actuator is not positioned at the shaft, the inertia/due to application of a mechanical external force to the magnetic disk drive unit, acts in such a direction as to rotate the head actuator, resulting in the tendency of the track to slip. A conventional means to prevent such track slip is, for example, providing a balancer weight at an end of the coil to thereby position the center of gravity of the head actuator at the shaft. However, the provision of a balancer weight at the end of the coil greatly increases the inertia of the head actuator making high-speed access difficult.